Platyisao holodividum,

Chiesa, Ignacio L. & Alonso, Gloria M., 2014, A new genus and species of Platyischnopidae (Amphipoda: Gammaridea) from the Argentine Sea, South-West Atlantic Ocean, Zootaxa 3811 (1): -1

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http://dx.doi.org/10.11646/zootaxa.3811.1.2

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scientific name

Platyisao holodividum
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Platyisao holodividum  gen et. sp. nov.

( Figures 1–7View FIGURE 1View FIGURE 2View FIGURE 3View FIGURE 4View FIGURE 5View FIGURE 6View FIGURE 7)

Material examined. Holotype: Puerto Quequén, Sta. 2, 38º 41.450 ’S, 58 º 42.101 ’W, 39.5 m, 15 Dec 2005, Rauschert sledge, colls B. Doti, D. Roccatagliata and I. Chiesa: 1 adult ♀, 2.5 mm (MACN-In 38293).

Paratypes: same locality as holotype: 7 adults ♀ (MACN-In 38294), MACN-In 38294 a used for the description. San Antonio Oeste, San Matías Gulf, colls B. Doti, D. Roccatagliata and I. Chiesa, 3 Jan 2005; Sta. 2, 40º 53.090 ’S, 65 º04.434’W, 12 m, van Veen grab: 16 adults ♂, 19 adults ♀, 6 specimens sex indet. (MACN-In 38295), MACN-In 38295 a used for SEM. Sta. 7, 40º 53.634 ’S, 65 º04.033’W, 19 m, van Veen grab: 11 ♂, 2 adults ♀, 2 specimens sex indet. (MACN-In 38296), MACN-In 38296 a used for SEM. Sta. 4, 40º 53.515 ’S, 65 º04.166’W, 15 m, van Veen grab: 21 ♂, 26 ♀, 26 specimens sex indet. (MACN-In 38297). Sta. 12, 40º 53.078 ’S, 65 º04.337’W, 17 m, Rauschert sledge: 2 ♂, 50 ♀ (MACN-In 38298). “Mejillón II” expedition, colls P. Penchaszadeh, G. Pastorino, D. Urteaga, M. Martínez and I. Chiesa; Sta. 1, 38°09.597’S, 57 ° 17.058 ’W, 35 m, 9 Sep 2009, small dredge: 1 ♀ (MACN-In 39112). Sta. 2, 38° 14.349 ’S, 57 ° 14.950 ’W, 38 m, 9 Sep 2009, Rauschert sledge: 7 ♀, 1 specimen sex indet. (MACN-In 39113). Sta. 18, 37° 42.644 ’S, 56 ° 33.409 ’W, 51 m, 14 Sep 2009, Rauschert sledge: 6 ♀, 5 specimens sex indet. (MACN-In 39114). Sta. 20, 37°09.091’S, 56 °07.967’W, 44 m, 14 Sep 2009, small dredge: 7 ♀, 4 specimens sex indet. (MACN-In 39115). Sta. 21, 37° 11.172 ’S, 56 °09.270’W, 33 m, 14 Sep 2009, Rauschert sledge: 3 ♀ (MACN-In 39116). Sta. 23, 36°07.662’S, 55 ° 33.177 ’W, 27 m, 15 Sep 2009, small dredge: 14 ♀, 10 specimens sex indet. (MACN-In 39117). Sta. 24, 36°08.746’S, 55 ° 51.937 ’W, 17 m, 15 Sep 2009, small dredge: 18 ♀ (MACN-In 39118). Sta. 25, 36° 17.217 ’S, 56 ° 29.867 ’W, 15 m, 15 Sep 2009, Rauschert sledge: 2 ♀ (MACN-In 39119).

Diagnosis. Same as for the genus.

Description of the adult female based on the holotype MACN-In 38293 (2.5 mm) and paratypes MACN-In 38294 a (2.4 mm) and MACN-In 38295 a (2.3 mm).

Head ( Figs. 1View FIGURE 1 A, B, C) 19.5 % of total body length, all surface covered with sensorial setae almost equidistant from each other, proximal and medial sections broad laterally, ventral margin undulated and concave at insertion of antennae, with 2 pores bearing 1 seta each ( Fig. 1View FIGURE 1 D). Rostrum conical ( Figs. 1View FIGURE 1 A, C), distally wrinkled (in specimens preserved in alcohol), with equatorial constriction, surface distally with distinct subrounded scales (almost of hexagonal shape), surface proximally almost smooth ( Figs. 1View FIGURE 1 C, D, E). Each pit placed in a circular arrangement ( Figs. 1View FIGURE 1 B, C, D) bearing 1 seta of flat form and distally clip-shaped ( Figs. 1View FIGURE 1 C, E), some pits have 2 small pores near the equatorial constriction ( Fig. 1View FIGURE 1 F). Eyes absent.

Antenna 1 ( Fig. 2View FIGURE 2 A), peduncular article 1 suboval and broad, 0.56 x as long as wide, 2.93 x wider than peduncular article 2, ventral and dorsal margins without setae, ventrodistal corner slightly expanded, with 4 short plumose setae (one longer), dorsodistal corner with 3 short simple setae (one shorter) and 1 medium plumose setae, lateral margin with 1 short simple seta ventrodistally; peduncular article 2 elongate and rectangular, 2.46 x as long as wide, 1.48 x as long as peduncular article 1, ventral and dorsal margins straight, without setae, ventrodistal corner weakly expanded, rounded, bearing 2 long simple setae and 4 short setae (one plumose), dorsodistal corner weakly expanded, with 1 short simple seta and 1 setule; peduncular article 3 about as long as article 1, about 2.1 x as long as wide, ventral and dorsal margins straight, without setae, lateral margin with 1 short plumose seta proximally, ventrodistal corner with 1 short simple setae and 1 setule, dorsodistal corner slightly expanded, rounded, bearing 4 short simple setae (2 shorter and thinner). Primary flagellum with 5 articles of similar length, 1.42 x as long as peduncle, articles 1–4 bearing short simple setae distally, article 5 with a few medium and thin simple setae apically, articles 2–4 bearing 1 long distal aesthetasc each; accessory flagellum short, with 2 articles, 0.32 x as long as primary flagellum, article 1 with 2 short setae (one plumose) dorsodistally, article 2 with a few medium thin simple setae apically.

Antenna 2 ( Fig. 2View FIGURE 2 B), considerably longer than antenna 1; peduncular articles 1–3 damaged during dissection (not illustrated); peduncular article 4 elongate, 3.33 x as long as wide, ventral margin almost straight, without setae, ventrodistal corner rounded, dorsal margin almost straight, with 1 medium simple seta proximally, and a subfacial medial row of 4 medium to long thin distally bifid setae, dorsodistal corner with 2 medium stout simple setae and 2 small distally bifid setae, lateral margin with a subfacial distal row of 5 medium to long distally bifid setae; peduncular article 5, about 0.83 x as long as peduncular article 4 and a bit narrower, ventral, dorsal, and lateral margins almost straight, without setae, ventrodistal and dorsodistal corners with 1 short simple seta each; flagellum 1.15 x as long as peduncular articles 4 and 5 combined, with 6 articles (1 and 2 the shortest), all articles with some short to medium thin simple setae distally.

Mandibles ( Figs. 2View FIGURE 2 C, D), both incisors of medium size, broad at the base and taperig towards the apex, smooth, right incisor conical, left incisor shorter, with apex slightly blunt bearing medial weak notch; right lacinia mobilis thin, weakly bidentate, teeth rounded, inner one a bit longer, left lacinia mobilis wider and longer, slightly flabellate, with 4 teeth, distal one conical and the largest; accessory setal rows absent; both molars covering more than half of medial mandibular surface, weakly triturative, right molar elongate and suboval, left molar of medium size and subovoid; palpar humps broad and elongate. Palps opposite to molar, article 1 short, subquadrate, 1.38 x as wide as long, without setae; article 2 about 3.33 x as long as article 1, broad (1.71 x as long as wide), inner margin slightly convex, with 1 medium simple seta subdistally; article 3 thin, elongate, 4.41 x as long as wide, and 1.25 x as long and 0.56 x as wide as article 2, both margins almost straight, without setae, apex oblique, with inner feeble cleft and 2 long (about as long as palp article 3) simple setae.

Maxilla 1 ( Fig. 3View FIGURE 3 A), inner plate of medium length, subrectangular, slightly thin, weakly concave on inner margin, rather blunt apically and with 1 medium stout simple seta; outer plate elongate, apical margin distinctly straight, with 7 ungrouped robust setae: 2 medium and bifid (with sharp branches) setae, 2 medium simple apically acute setae, and the 3 outermost setae larger, stout, and slightly curved (2 apically rounded and 1 denticulate); palp uniarticulate, 2.36 x as long as wide, reaching half of outer plate, apex blunt bearing 2 very long plumose setae.

Maxilla 2 ( Fig. 3View FIGURE 3 B), both plates extending subequally; inner plate thin, apical margin subrounded, with 4 setae: 2 long and slender simple setae, 1 short simple seta, and 1 medium plumose seta almost placed on inner margin; outer plate subrectangular, much wider than inner plate, apical margin straight, with 8 setae: 6 stout and long simple setae, 1 long plumose seta, and 1 short simple seta almost placed on lateral margin.

Maxilliped ( Fig. 3View FIGURE 3 C), inner plate short, of medium width, subrectangular, truncate apically, with 4 medium to long stout plumose setae distally (inner one the shortest); outer plate medium to elongate (almost reaching palp article 3), broad, suboval, lateral margin smooth, medial margin slightly undulated, with 2 short to medium simple setae proximally and 6 spaced robust setae mediodistally: three proximal medium and bearing a comb-like hairs on inner margin, and three distal stouter, increasing in length and strongly serrated on inner margin. Palp 4 -articulate, large, broad basally, article 1 without setae; article 2 elongate, of medium width, subrectangular, medial margin undulated, with 9 long simple setae; article 3 of medium length, 0.81 x as long as article 2, medial margin with strong excavation subdistally and expanded into a rounded lobe overreaching the base of article 4, bearing 4 long simple setae (3 subproximally and 1 medially), and 2 long simple setae subdistally, outer margin without setae; article 4 short, stout, 0.36 x as long as article 3, apically rounded with 1 thin medium simple seta and 1 long (0.84 x as long as article 4) straight nail.

Coxae 1–4 ( Fig. 1View FIGURE 1 A) progressively longer and wider, ventral margins forming a near continuous line, strongly overlapping; coxa 4 greatly covering coxa 3.

Gnathopod 1 ( Figs. 4View FIGURE 4 A, B), subchelate, coxa elongate, subrectangular, 2.1 x as long as wide, anterior margin almost straight, anteroventral corner slightly expanded and subacute, ventral margin somewhat convex, with 1 setule, posteroventral corner slightly rounded, with 1 long simple seta and 1 setule, posterior margin almost straight, with 2 subfacial medium simple setae; width ratio of basis-ischium-merus-carpus-propodus = 19: 19: 17: 25: 32, length ratio = 110: 19: 27: 52: 34; basis very thin and elongate (5.78 x as long as wide), somewhat expanded distally, anterior margin slightly concave medially, posterior margin slightly convex, with 1 long simple seta on distal third and 1 medium simple seta on posterodistal corner; ischium short, subquadrate, with 1 long simple seta on posterior margin; merus subtriangular, posterior margin convex and much longer than anterior margin, with 1 medium simple seta on posterodistal corner; carpus pear-shaped, of medium length, 2 x as long as wide and 1.53 x as long as propodus, posterior margin with 1 medium simple seta medially, posterodistal corner expanded and rounded, with 5 medium to long simple setae, anterior margin slightly convex, almost straight distally; propodus subchelate, suboval, somewhat longer than wide, 1.28 x as wide as carpus, with some facials short to medium simple setae, anterior margin slightly convex, anterodistal corner with a group of medium simple setae, posterior margin rounded, shorter than anterior margin, with 6 short to medium simple setae distally, palmar corner subacute, with 2 stout setae, palm oblique, slightly shorter than anterior margin, sinuous and denticulate proximally, with 1 stout seta near palmar corner and 2 groups of facial and subfacial short to medium simple setae (proximal group on a protuberance); dactylus of medium length and curved, reaching palmar corner, with 1 short simple seta anteromedially, and 2 short simple setae on inner margin.

Gnathopod 2 ( Figs. 4View FIGURE 4 C, D) similar but longer than gnathopod 1, subchelate; coxa elongate (approximately as long as coxa 1), subrectangular, 2.1 x as long as wide, anterior margin slightly concave, anteroventral corner somewhat expanded and subacute, ventral margin almost straight, with 2 proximal setules, posteroventral corner almost rounded, with 1 medium to long simple seta and 1 setule, posterior margin slightly convex, with 3 subfacial medium simple setae; width ratio of basis-ischium-merus-carpus-propodus = 25: 18: 19: 20: 31, length ratio = 144: 23: 25: 101: 45; basis very thin and elongate, 1.13 x as long as coxa 2 and 5.76 x as long as wide, distally somewhat expanded, anterior margin slightly concave medially, posterior margin slightly convex, with 1 very long simple seta on distal third and 1 long simple seta on posterodistal corner; ischium short, subrectangular, with 1 long simple seta on posterior margin; merus subtriangular, posterior margin slightly convex and much longer than anterior margin, with 1 short simple seta on posterodistal corner; carpus elongate, thin, subrectangular, 1.94 x longer than carpus of gnathopod 1, about 5.05 x as long as wide and 2.24 x as long as propodus, posterodistal corner expanded and subrounded, with 5 small to medium simple setae, anterodistal corner with 1 medium simple seta; propodus subchelate, suboval, 1.45 x as long as wide, slightly larger than that of gnathopod 1, anterior margin more straight than in gnathopod 1, with 3 subfacial medium simple setae, anterodistal corner with a group of medium simple setae, posterior margin convex, somewhat shorter than anterior margin, with 4 medium simple setae subdistally, palm oblique, shorter than anterior margin, sinuous and denticulate proximally, with 2 groups of a few subfacial short to medium simple setae, palmar corner subacute, with 2 medium stout setae (one larger); dactylus medium length and curved, almost reaching palmar corner, with 1 short simple seta anteromedially and 2 short simple setae on inner margin.

Pereopods 3 and 4 of similar shape, except for coxae.

Pereopod 3 ( Figs. 5View FIGURE 5 A, B), coxa subrectangular, 2 x as long as wide, anterior and posterior margins almost straight, subparallel, anteroventral corner slightly expanded and concave, ventral margin straight, with 1 setule anteriorly, posteroventral corner subrounded, with 1 setule and 1 medium simple seta, posterior margin without setae; length ratio of basis-ischium-merus-carpus-propodus-dactylus = 75: 14: 50: 24: 40: 24; basis of medium length, 3.26 x as long as wide, subrectangular, 0.79 x as long as coxa, posterior margin slightly convex, with 3 long simple setae, posterodistal corner with 1 medium to long simple seta; ischium subquadrate, posterodistal corner with 1 medium to long simple seta; merus of medium width, 2.27 x as long as wide, anterior margin convex, with 1 setule distally, posterior margin with 2 medium to long simple setae distally; carpus short, of medium width, 1.5 x as long as wide and 0.6 x as long as propodus, ventral margin subrounded, posterior margin with 1 setule and 1 medium simple seta subdistally, main apical robust seta (0.78 as long as carpus) strongly bifid distally and extending to 47 % length of propodus; propodus rectangular, thinner than carpus, 4.4 x as long as wide, with 5 robust setae on posterodistal surface (3 medium length bifid and 2 shorter simple); dactylus almost straight, thin, elongate, 0.6 x as long as propodus, medial face without setae and with weak notch distally.

Pereopod 4 ( Figs. 5View FIGURE 5 C, D), coxa greatly enlarged, 1.1 x as wide as long, anterior and posterior margins subparallel, anterior margin almost straight, anteroventral corner somewhat expanded, ventral margin very long and slightly convex, posterior margin medially rounded, posterodorsal section oblique and posteroventral section slightly rounded, posteroventral corner almost rounded; length ratio of basis-ischium-merus-carpus-propodusdactylus = 75: 15: 50: 25: 43: 25; basis of medium length, 3.13 x as long as wide, subrectangular, 0.68 x as long as coxa, posterior margin slightly convex, with 6 long simple setae (distal one the shortest); ischium subquadrate, posterodistal corner with 1 medium to long simple seta; merus of medium width, 2.17 x as long as wide, anterior margin convex, with 1 setule distally, posterior margin with 2 medium to long simple setae distally; carpus short, 1.56 x as long as wide and 0.58 x as long as propodus, ventral margin subrounded, posterior margin with 1 medium simple seta and 1 setule subdistally, main apical robust seta (0.81 x as long as carpus) distally bifid and extending to 56 % length of propodus; propodus rectangular, thinner than carpus, 4.7 x as long as wide, with 5 robust setae on posterodistal surface (3 of medium length bifid and 2 shorter simple); dactylus slightly curved, thin, elongate, 0.58 x as long as propodus, medial face without setae and with weak notch distally.

Pereopods 5–7 ( Figs. 1View FIGURE 1 A; 6 A, B, C) of similar shape, increasing in length from 5 to 7. Coxa and basis of pereopod 5 covering those of pereopod 6, and the latter those of pereopod 7.

Pereopod 5 ( Fig. 6View FIGURE 6 A), coxa with anterodorsal corner greatly expanded, anteromedial margin concave, ventral margin rounded, reaching to 70 % length of basis, with 3 notches (one very weak) bearing 1 setule each, anteroventral margin with 1 medium simple seta and 1 setule, posterior margin slightly convex, 3 x longer than anterodorsal margin; width ratio of basis-merus-carpus-propodus-dactylus = 40: 34: 20: 7: 1, length ratio = 59: 25: 35: 41: 29; basis pear-shaped, 1.47 x as long as wide, without facial ridge, of medium width basally and distal corners slightly expanded, posteroventral margin rounded and with 2 notches, anteroventral corner subacute, with 1 medium simple seta; merus and carpus of medium width, anterior, posterior and ventral margins with notches and several small, medium and large robust setae; merus 1.36 x as long as wide, anterior margin almost straight and posterior margin expanded and rounded; carpus subrectangular and thinner, 1.75 x as long as wide than merus, with 2 notches on both margins; propodus narrow, 5.85 x as long as wide, with 1 medial notch on both margins, bearing small to medium robust setae; dactylus elongate and thin, 0.7 x as long as propodus, bearing 1 proximal plumose setule on posterior margin.

Pereopod 6 ( Fig. 6View FIGURE 6 B), coxa similar to that of pereopod 5 but slightly larger, reaching to 64 % length of basis, posterior margin 2.7 x longer than anterodorsal margin, with 4 notches bearing setules, ventral margin with 1 medium and 3 short (one longer) simple setae; width ratio of basis-merus-carpus-propodus-dactylus = 60: 45: 28: 8: 2, length ratio = 70: 41: 53: 59: 34; basis subovoid, 1.16 x as long as wide, without facial ridge, posterior margin with tiny notches bearing setules, anterior margin with 2 short setae, anteroventral corner subacute and bearing 1 short and 1 medium simple setae; merus and carpus considerably wider and longer than in pereopod 5 but with robust setae of similar size and arrangement; merus 0.91 x as long as wide, anterior margin slightly undulated and posterior margin with 2 notches; carpus 1.89 x as long as wide, with 3 notches on both margins; propodus narrow, 7.37 x as long as wide, longer than in pereopod 5 (1.44 x), with 2 notches on both margins, bearing short to medium robust setae; dactylus elongate and thin, 0.57 x as long as propodus, bearing 1 proximal plumose setule on posterior margin.

Pereopod 7 ( Fig. 6View FIGURE 6 C), coxa short and wide, subovoid, 2.61 x as wide as long, ventral margin slightly convex, with 3 notches bearing setules; width ratio of basis-merus-carpus-propodus-dactylus = 73: 36: 23: 8: 2, length ratio = 71: 39: 46: 61: 43; basis very large, subovoid, 1.03 x as wide as long, without facial ridge, anterior and posterior margins divergent, lacking posterodistal tooth or deep notch, anterior margin convex, slightly undulated, bearing short and medium setae, posterior and ventral margins slightly rounded and undulated (with distinct small notches bearing setules), posteroventral corner slightly expanded and rounded, reaching to 28 % length of merus; merus and carpus large but smaller than in pereopod 6, bearing fewer and shorter robust setae; merus 1.08 x as long as wide; carpus 2 x as long as wide; propodus longer than in pereopod 6, very narrow, 7.63 x as long as wide, both margins with 2 weak notches bearing shorter robust setae; dactylus very long and thin, straight, 0.7 x as long as propodus, and 1.26 x as long as dactylus of pereopod 6, bearing 1 proximal plumose setule on posterior margin.

Pereon and pleon ( Fig. 1View FIGURE 1 A), surface with small pits bearing a setule each.

Pleonites 1–3 ( Fig. 1View FIGURE 1 A) smooth dorsally.

Epimeral plate 1 ( Fig. 1View FIGURE 1 A) shorter than epimeral plates 2 and 3, anterior margin oblique, anteroventral corner rounded, ventral margin slightly convex, posteroventral corner distinctly rounded, posterior margin concave medially and with 2 very weak notches bearing setules.

Epimeral plate 2 ( Fig. 1View FIGURE 1 A) with anteroventral corner rounded, ventral margin almost straight and without setae, posteroventral corner strongly produced and acute, posterior margin oblique, slightly sinuous and without setae.

Epimeral plate 3 ( Fig. 1View FIGURE 1 A) subequal in length to epimeral plate 2, dorsolateral margin smooth, anterior margin slightly convex, ventral margin somewhat rounded, posteroventral corner rounded and produced into a very small tooth, posterior margin slightly concave distally and with a strong constriction proximally.

Urosomite 1 elongate, with 1 medium robust setae near to the base of uropod 1.

Urosomite 2 longer than broad, dorsal margin short, ventral margin oblique.

Urosomite 3 as broad as long, posterior margin undulated and posteroventral corner with 1 medium robust seta.

Uropods, surface of peduncles and rami covered with comb-shaped flakes of tiny hairs arranged in rows ( Fig. 1View FIGURE 1 G). Uropod 1 longer than uropod 2, uropod 3 very elongate; telson longer than peduncle of uropod 3.

Uropod 1 ( Fig. 7View FIGURE 7 A), peduncle elongate, 3.33 x longer than broad, dorsomedial margin with 3 long robust setae, ventral margin with 1 short proximal robust seta, distolateral corner expanded into a curved tooth, with 1 subfacial medium and stout robust seta; outer ramus slightly shorter (0.85 x) and inner ramus somewhat longer (1.13 x) than peduncle, both rami expanded into a small and curved distal tooth; outer ramus with 1 dorsolateral medium robust seta and 4 subapical robust setae: 3 medium (one slightly curved distally) and 1 long; inner ramus 1.32 x as long as outer ramus, with 1 dorsomedial robust seta and 5 subapical robust setae: 3 short (one slightly curved distally), 1 medium and 1 long.

Uropod 2 ( Fig. 7View FIGURE 7 B), peduncle 0.57 x as long as peduncle of uropod 1, distolateral margin produced into a triangular flap, dorsomedial margin with 3 long robust setae (1 placed medially and 2 on distal corner), apicolateral corner with 1 long robust seta dorsally, ventral margin pilose, without robust setae; outer ramus somewhat shorter (0.94 x) and inner ramus longer (1.31 x) than peduncle, both rami expanded into a small and curved distal tooth; outer ramus with 1 medium robust seta in notch on dorsomedial margin and 4 subapical robust setae: 3 medium (1 slightly curved distally) and 1 long; inner ramus 1.4 x as long as outer ramus, with 1 long robust seta in notch on dorsomedial margin and 4 subapical robust setae: 3 medium and 1 long.

Uropod 3 ( Fig. 7View FIGURE 7 C), peduncle very short, 0.38 x as long as peduncle of uropod 1, with 2 long distolateral robust setae, 2 long distoventral robust setae, and 1 subdistal short robust seta on inner weak protuberance; inner ramus very short, 0.75 x as long as peduncle, subtriangular, with 1 medial short simple seta on inner margin and 1 setule distally; outer ramus very elongate and broad, biarticulated, 5.2 x as long as peduncle and 1.77 x as long as inner ramus of uropod 1, article one with 3 notches on lateral margin, each one bearing 2 medium robust setae, and 2 short robust setae on distal corner, inner margin with 2 subdistal short robust setae and 2 robust setae (1 short and 1 medium) on distal corner; article 2 of medium width, short, 0.32 x as long as article 1, without setae on inner and lateral margins, distal corners expanded and pointed, with 2 setules, and 1 long and thin simple seta apically.

Telson ( Fig. 7View FIGURE 7 D) about 1.47 x as long as broad, completely cleft, margins almost straight, both lobes bearing 2 short penicillate setae on dorsomedial outer margin; apex of each lobe produced into 2 small sharp teeth (medial tooth the longest), with 2 short robust setae dorsally (medial seta the longest).

Male observations. In males of Platyisao holodividum  gen. et. sp. nov., the posteroventral corner of the epimeral plate 2 is expanded but slightly rounded and the urosome ( Fig. 1View FIGURE 1 H) resembles that of females [based on the male paratypes MACN-In 38295 b (2.2 mm) and MACN-In 38296 a (1.9 mm)].

Etimology. The specific name holodividum  , is combined from the Greek holos meaning totality and the Latin dividum meaning divided, alluding to the completely cleft telson.

Type locality. Puerto Quequén, 38 º 41.450 ’S, 58 º 42.101 ’W, 39.5 m.

Geographical distribution. Platyisao holodividum  gen. et. sp. nov. is distributed from off Bahía San Borombón (Buenos Aires province) to San Antonio Oeste (San Matías Gulf, Patagonia), along the northern Argentine Sea (see Fig. 8View FIGURE 8).

Bathymetric range. 12 to 51 m.

Habitat. In San Antonio Oeste Platyisao holodividum  gen. et. sp. nov. was found on fine and coarse sand bottoms; at Puerto Quequén in fine sand mixed with mud; and at the stations of the “Mejillón II” expedition (Buenos Aires province coast) in fine and coarse sand often containing shell fragments.

Remarks. Platyisao  gen. nov. conforms to the diagnostic characters defining the family Platyischnopidae  , including: a cone-shaped rostrum; a large and weakly triturative molar; fossorial pereopods, with the posterior ones being elongate and pereopod 7 the longest; the uropod 3 has an elongate and biarticulate outer ramus and a very short inner ramus ( Barnard & Drummond 1979; Barnard & Karaman 1991).

In addition to the above-mentioned family features, Platyisao  gen. nov. shares: the absence of eyes with Indischnopus  ; the article 2 of antenna 1 elongate with Platyischnopus  ; the accessory flagellum of antenna 1 short with Indischnopus  and Platyischnopus  ; the mandibles without accessory setal row with Skaptopus  ; the palp of maxilla 1 uniarticulate with Skaptopus  and Tiburonella  ; the coxa 1 rectangular with Eudevenopus  , Indischnopus  , Skaptopus  and Tiburonella  ; the uropod 3 outer ramus article 2 without setae on inner and lateral margins with Indischnopus  , Skaptopus  and Tomituka  (unknown for Yurrokus  ); and the posteroventral corner of the epimeral plate 2 strongly produced and acute with Indischnopus  and Skaptopus  .

Platyisao  gen. nov. differs from Eudevenopus  by: outer plate of maxillipeds broad and with 6 robust setae (thin and with less setae in Eudevenopus  ); carpus of gnathopods longer than propodus (subequal or shorter); basis of pereopod 5 of medium width (narrow).

Platyisao  gen. nov. differs from Indischnopus  by: pleonite 3 dorsolaterally smooth and posteroventrally rounded (with dorsolateral teeth and posteroventral upturned tooth in Indischnopus  ).

Platyisao  gen. nov. differs from Platyischnopus  by: peduncle article 2 of antenna 1 without setae (with rows of ventral and dorsal setae in Platyischnopus  ); ischium of gnathopods short (elongate); merus and carpus of pereopod 7 of medium width (very broad); epimeral plate 2 posteroventral corner strongly produced and acute (slightly extended).

Skaptopus  is the unique genus of Platyischnopidae  without the cone-shaped rostrum, and the pleon is toothed instead of dorsally smooth.

Platyisao  gen. nov. differs from Tiburonella  by: peduncle article 2 of antenna 1 elongate and narrow (short and broad in Tiburonella  ); peduncle article 4 of antenna 2 without ventral setae (with several setae); outer plate of maxilliped broad and with 6 robust setae (thin and with less setae); carpus of gnathopods longer than propodus (slightly shorter).

Platyisao  gen. nov. differs from Tittakunara  by: accessory flagellum of antenna 1 biarticulated (12 -articulated in Tittakunara  ); peduncular article 4 of antenna 2 without ventral setae (with several ventral groups of dense long setae); coxa 1 elongate and rectangular (very short and with expanded anteroventral margin); inner ramus of uropodo 3 very short (of medium length).

Platyisao  gen. nov. differs from Tomituka  by: peduncle article 2 of antenna 1 elongate and narrow (shorter and wider in Tomituka  ); peduncular article 4 of antenna 2 without ventral setae (with several groups of long ventral setae); coxa 1 rectangular (strongly expanded anteroventrally).

Platyisao  gen. nov. differs from Yurrokus  by: peduncle article 2 of antenna 1 elongate and narrow (shorter and wider in Yurrokus  ); peduncular article 4 of antenna 2 without ventral setae (with some groups of long ventral setae); coxa 1 elongate and rectangular (short and with anteroventral margin expanded); carpus of gnathopod 2 much longer than propodus (somewhat longer).

The new genus Platyisao  exhibits unique features that clearly distinguish it from the remaining genera of Platyischnopidae  , as follows: the gnathopods subchelate; the coxa 4 with posterior margin medially rounded (not tapering) and distal section oblique; the pereopod 7 with basis very large, anterior and posterior margins divergent, posteroventral margin slightly rounded and undulated (with small notches); and telson elongate, completely cleft, with each apex produced into to 2 small sharp teeth.